Transcript Serial Communications

Serial Communications
Sending and receiving data between
devices
CE-2810
Dr. Mark L. Hornick
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Serial Communications describes a
general way in which two devices can
send or receive data from one another
To send or receive data, bits
have to be moved around
The term Serial indicates that
the data bits are transmitted
sequentially, one bit at a time
Conversely, in parallel communication,
multiple bits are transmitted
simultaneously
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Dr. Mark L. Hornick
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The physical signaling of serial
data takes many forms
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USB (Universal Serial Bus)
Firewire
Bluetooth
IR
Ethernet
RS-232
RS-422
RS-485
WiFi
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Atmel Atmega32
USART Subsystem
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USART overlaps PortD pins
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RXD pin is used to receive serial
data
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Enabling the USART receiver
overrides PD0 function
TXD pin is used to transmit serial
data
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Enabling the USART transmitter
overrides PD1 function
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Dr. Mark L. Hornick
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RS-232 on the SunRom board
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The SunRom board includes a RS-232
converter chip that converts the low-level
signals of TXD/RXD to RS-232 voltages
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TXD/RXD: 0 to +5v
RS-232: -15 to +15v
The converter chip
interfaces the
Atmega32 to the
15-pin D-shell
connector
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Dr. Mark L. Hornick
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TTL vs RS-232 voltage levels
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TTL (transistor logic) signals
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v < 2 = logical 0
v > 3 = logical 1
2 < v < 3 = undefined
RS-232 signals
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-15 < v < -3 = logical 1
+3 < v < +15 = logical 0
-3 < v < +3 = undefined
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Dr. Mark L. Hornick
Note that logic values
appear to be inverted if
RS-232 signals are
viewed on an
oscilloscope
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Only 3 lines on the 15-pin
connector are used for RS-232;
Tx(2), Rx(3), and Ground(5)
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Connection between two
communicating serial devices
The Tx (transmit) line of one device is
connected to the Rx (receive) line of the
second device
Tx
Tx
Rx
Rx
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Asynchronous vs Synchronous
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In synchronous communications, the two
communicating devices must keep their
respective clock signals synchronized
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Even when no data is being sent, the two devices
continuously exchange “sync” characters
In asynchronous communications, the two
devices maintain their own clocks
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Nothing is transmitted when there is nothing to send
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But a “handshake” has to be established whenever
data needs to be transmitted – extra overhead
The USART subsystem of the Atmega32 supports both
modes, but we’ll only use the asynchronous mode.
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Dr. Mark L. Hornick
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Asynchronous transmission of
a byte of data
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Each byte of data is surrounded by “framing”
bits, inserted by the USART
LSB
MSB
5,6,7,8, or 9 data bits
Parity bit can be disabled (no parity bit)
1 or 2 stop bits
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The Parity bit is an error
checking mechanism used to
detect data corruption
Equal to 1 if the number of non-zero data bits is odd,
(#data bits + parity bit = even number of bits)
Equal to 1 if the number of non-zero data bits is even,
(#data bits + parity bit = odd number of bits)
The Parity is computed first on the transmitting end, and included as the Parity bit in the frame. The
Parity is then recomputed on the receiving end, and the result is compared to the Parity bit value. If the
Parity calculation on the receiving end does not match, the receiver knows data corruption occurred.
This allows errors to be DETECTED, but does not provide a means of CORRECTION.
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RS-232 signal transmission of
the letter ‘K’ (ASCII 0x4C)
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Speed metrics
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The speed of serial transmission is usually
measured in bits-per-second
This is called the baud rate
Because of the overhead bits used in each
frame, 2400 baud is NOT the same as 300
characters per second
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Since each character takes >8 bits to transmit
The Atmega32 USART can be configured to
operate between 2400 and 1M baud
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